In aircraft braking systems it has always been desirable that inadvertent braking be prevented. In the mechanical braking systems of the past, the likelihood of inadvertent braking has been minimal, since brake metering valves simply do not fail in a "pressure applied" or braking mode. However, the advent of electrical or "brake-by-wire" systems has given rise to concerns that a hardware or software failure could result in undesired brake application. A primary concern in such systems is that a software fault might inadvertently apply brakes. In such brake-by-wire systems, the prior art has taught the use of dual software and dissimilar hardware approaches to achieving this redundancy. However, such approaches increase the cost of the braking systems and necessarily decrease their reliability Since the redundancy necessarily requires the implementation of more hardware and/or software, the maintenance problems are aggravated. It is also known to employ a single software system with a hardware monitor to set the hydraulic pressure at the brake and provide appropriate responsive action with respect thereto, but such systems suffer the same shortcomings, and which enables the brake valves with hardware.
There is a need in the art for a simple fail safe brake-by-wire system for aircraft that does not aggravate the reliability factor by significantly increasing the amount of hardware and software employed.